Hi Timur, I believe you see the space for the compressed class data (Compressed class space), which is by default 1GB and controlled by -XX:CompressedClassSpaceSize . I think this area is semantically one subarea of the metaspace, the other being non-class data.
I am unsure about the difference between MaxMetaSpaceSize and CompressedClassSpaceSize, but what I believe is that: - MaxMetaSpaceSize limits the amount of total committed (not reserved) space - CompressedClassSpaceSize sets the amount of space to be reserved (not committed) for compressed classes only. This would fit your NMT output, which says: "Class (reserved=1142808KB, committed=106520KB)" so, ~100M are committed. With a MaxMetaSpaceSize=256M - what you specified - you would not be able to commit the whole 1GB, but hit the limit far earlier. See also: https://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/considerations.htm (the section about UseCompressedClassesPointers) Please take this with a grain of salt. I am not sure I understand this correctly and would value input from other developers. Kind Regards, Thomas On Thu, Feb 4, 2016 at 2:13 PM, Timur Akhmadeev <timur.akhmad...@gmail.com> wrote: > Hi, > > I'm trying to figure out which parts of memory Hotspot can allocate in > HugePages on Linux (appears to be heap and code cache by default, plus > Metaspace with -XX:+UseLargePagesInMetaspace) and noticed something I can't > explain. > > There's 1G allocation on the OS level in 4K pages that is apparently > attributed to Class metadata, although Metaspace is maxed at a value much > lower than 1G. Question: why? > > [oracle@oel6u7 bin]$ grep -B 11 'KernelPageSize: 4 kB' > /proc/18906/smaps | grep '^Size:' | head > Size: 4 kB > Size: 4 kB > Size: 2764 kB > Size: 12288 kB > Size: 1036288 kB > Size: 128 kB > Size: 4 kB > Size: 4 kB > Size: 4 kB > Size: 8 kB > > [oracle@oel6u7 bin]$ /home/oracle/jdk1.8.0_72/bin/jcmd 18906 > VM.native_memory summary > 18906: > > Native Memory Tracking: > > Total: reserved=2069703KB, committed=1032559KB > - Java Heap (reserved=786432KB, committed=786432KB) > (mmap: reserved=786432KB, committed=786432KB) > > - Class (reserved=1142808KB, committed=106520KB) > (classes #13624) > (malloc=2072KB #22966) > (mmap: reserved=1140736KB, committed=104448KB) > > - Thread (reserved=56009KB, committed=56009KB) > (thread #55) > (stack: reserved=55512KB, committed=55512KB) > (malloc=178KB #272) > (arena=319KB #108) > > - Code (reserved=55407KB, committed=55407KB) > (malloc=5487KB #7167) > (mmap: reserved=49920KB, committed=49920KB) > > - GC (reserved=2600KB, committed=1748KB) > (malloc=32KB #355) > (mmap: reserved=2568KB, committed=1716KB) > > - Compiler (reserved=213KB, committed=213KB) > (malloc=82KB #331) > (arena=131KB #3) > > - Internal (reserved=4921KB, committed=4917KB) > (malloc=4885KB #16137) > (mmap: reserved=36KB, committed=32KB) > > - Symbol (reserved=17518KB, committed=17518KB) > (malloc=14377KB #172159) > (arena=3141KB #1) > > - Native Memory Tracking (reserved=3617KB, committed=3617KB) > (malloc=149KB #2329) > (tracking overhead=3468KB) > > - Arena Chunk (reserved=178KB, committed=178KB) > (malloc=178KB) > > [oracle@oel6u7 bin]$ /home/oracle/jdk1.8.0_72/bin/jcmd 18906 > VM.native_memory detail | grep -A 10 'for Class' > [0x0000000100000000 - 0x0000000140000000] reserved 1048576KB for Class from > [0x00007f71950e2ad2] ReservedSpace::initialize(unsigned long, unsigned > long, bool, char*, unsigned long, bool)+0xc2 > [0x00007f71950e2d0b] ReservedSpace::ReservedSpace(unsigned long, > unsigned long, bool, char*, unsigned long)+0x1b > [0x00007f7194ec8300] > Metaspace::allocate_metaspace_compressed_klass_ptrs(char*, unsigned > char*)+0x40 > [0x00007f7194eca7df] Metaspace::global_initialize()+0x4cf > > [0x0000000100000000 - 0x0000000100c00000] committed 12288KB from > [0x00007f71950e2549] VirtualSpace::expand_by(unsigned long, > bool)+0x199 > [0x00007f7194ec6766] > VirtualSpaceList::expand_node_by(VirtualSpaceNode*, unsigned long, unsigned > long)+0x76 > [0x00007f7194ec9580] VirtualSpaceList::expand_by(unsigned > long, unsigned long)+0xf0 > [0x00007f7194ec9713] VirtualSpaceList::get_new_chunk(unsigned > long, unsigned long, unsigned long)+0xb3 > ... > > [oracle@oel6u7 bin]$ head ../logs/gc.log > Java HotSpot(TM) 64-Bit Server VM (25.72-b15) for linux-amd64 JRE > (1.8.0_72-b15), built on Dec 22 2015 22:00:07 by "java_re" with gcc 4.3.0 > 20080428 (Red Hat 4.3.0-8) > Memory: 4k page, physical 3082784k(506980k free), swap 1257468k(1148276k > free) > CommandLine flags: -XX:InitialCodeCacheSize=50331648 > -XX:InitialHeapSize=49324544 -XX:MaxHeapSize=805306368 > -XX:MaxMetaspaceSize=268435456 -XX:MetaspaceSize=268435456 > -XX:NativeMemoryTracking=detail -XX:+PrintGC -XX:+PrintGCTimeStamps > -XX:ReservedCodeCacheSize=50331648 -XX:+UseCompressedClassPointers > -XX:+UseCompressedOops -XX:+UseLargePages -XX:+UseLargePagesInMetaspace > > -- > Regards > Timur Akhmadeev > > _______________________________________________ > hotspot-gc-use mailing list > hotspot-gc-use@openjdk.java.net > http://mail.openjdk.java.net/mailman/listinfo/hotspot-gc-use > >
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